It is known that the efficiency of a combustion turbine engine improves as the combustion gas firing temperature increases. However, as the firing temperature increases the high temperature durability of turbine components must be correspondingly increase. Although nickel and cobalt based superalloy materials may be used for components in the hot gas flow path, such as combustor transition pieces and turbine rotating blades and stationary vanes, even these superalloy materials are not capable of surviving long-term operation at temperatures that sometimes exceed 1,400 degrees C.
In many applications a metal substrate (e.g., of a component) or a bond coating overlying the metal substrate is coated with a ceramic insulating material, such as a thermal barrier coating (TBC), to reduce the operating temperature of the metal substrate and the magnitude of the temperature transients to which the metal is exposed.
TBCs have played a significant role in reducing the operating temperature of turbine components and in realizing improvements in turbine efficiency. Obviously the thermal barrier coating protects the substrate only while the coating remains substantially intact on the substrate surface.
During operation, the TBCs and any underlying bond coatings are subject to spallation and degradation. The cause of such distresses may include: high physical stresses caused by high-velocity ballistic impacts by foreign objects, differential thermal expansion (i.e., between the underlying superalloy substrate and overlying bond coating or between the bond coating and the TBCs), material defects, and material property changes due to the operating environment. Any of these situations can lead to damage and even total removal of the bond coating and/or the TBC from the substrate surface. The conventional repair process involves stripping the damaged layer(s) and recoating the substrate, a time-consuming and costly task.
It is known to control a roughness parameter of a surface (such as the substrate surface) to improve adhesion of an overlying bonding layer or a thermal barrier coating. U.S. Pat. No. 5,419,971 describes a laser ablation process where removal of material by direct vaporization (e.g., without melting the material) is purportedly used to form three-dimensional features at the irradiated surface. See col. 6, line 3. These features are limited to patterns formed within the irradiated surface. These laser ablation processes do not form structures extending beyond the surface. Thus processes that can provide improved structural formations conducive to enhanced adhesion are needed.